Thin film resistors are typically created by depositing a thin film constructed of a metal alloy upon a substrate. Specifically, the thin film is deposited upon a substrate using a sputtering deposition process. Thin film resistors are found in laminate substrates used for electronic applications such as, for example, printed circuit boards.
A thin film resistor may be attached to a top surface of the electronic substrate or, alternatively, embedded within the laminate substrate. When embedded in a substrate, the thin film resistor is referred to as an embedded thin film resistor. Embedded thin film resistors are typically composed of nickel-chromium alloys such as, for example, nickel chromium aluminum silicon (NiCrAlSi). The nickel-chromium alloy is deposited upon a copper foil. The nickel-chromium alloy and the copper foil are then bonded to a dielectric layer using a lamination process, where the copper foil acts as a conductive layer and the nickel-chromium alloy acts as a resistive layer. The resistive layer is then etched to define discrete embedded thin film resistors. Any number of etching process may be used to define the embedded thin film resistors, however, the specific etching process is selected based on the specific metals included in the nickel-chromium alloy.
The resistance variation of a thin film resistor depends upon several factors. Specifically, physical properties such as, for example, film thickness, residual stress, uniformity, and surface roughness of the conductive layer may affect the resistance variation of the thin film resistor. Moreover, the sheet resistance of the nickel-chromium alloy before etching is inversely proportional to thickness. The thickness of the nickel-chromium layer is determined based on sputtering parameters such as linear speed and power, however, the sputtering parameters may be difficult to control. Additionally, the resolution and specific type of etching used to create the discrete resistor elements also affects the resistance variations. Finally, the temperature and cycle time of the lamination process also affects the resistance of the nickel-chromium alloy. Thus, it is challenging to control the resistance variation of a thin film resistor.